The present invention relates to a spark-ignition direct injection engine.
In view of improving the theoretical thermal efficiency in spark-ignition gasoline engines, it is effective to improve the geometric compression ratio thereof. For example, JP2007-292050A discloses a spark-ignition direct injection engine with a high compression ratio where its geometric compression ratio is set to 14:1 or above.
JP2009-091994A discloses an art of avoiding knock in a spark-ignition direct injection engine where compression-ignition combustion and spark-ignition combustion are switched therebetween according to an operating state of the engine, by implementing EGR and enriching an air-fuel ratio when transiting from the compression-ignition combustion to the spark-ignition combustion.
Meanwhile, such a spark-ignition gasoline engine with a high compression ratio disclosed in JP2007-292050A is advantageous in improving the thermal efficiency; however, it has a problem of easily causing knock (end gas knock) when the operating state of the engine is within a middle or high engine load range. As disclosed in JP2007-292050A, it is generally known to retard an ignition timing as a countermeasure against knock; however, the retarded ignition timing will cause a reduction of the torque.
Further, in vehicles which are set to use fuel with a high octane number (e.g., high-octane gasoline or premium gasoline) (i.e., a vehicle specified for high octane fuel), since the fuel has a high anti-knock quality, the ignition timing is set to advance so as to increase the torque. However, if fuel with a low octane number (e.g., regular gasoline) is supplied to the high octane vehicle, knock will occur more easily, and therefore, a knock control of retarding the ignition timing to avoid knock will intervene, but when the knock control intervenes, the torque decreases.